feat: Replace SSE with WebSocket for UI notifications (#36965)

* Closes #36942
* Fixes #19265 

Replaces the SSE-based push channel (`/user/events`) with a WebSocket
endpoint (`/-/ws`).

### What changes

- **New `/-/ws` endpoint** (authenticated). One WebSocket per origin,
shared across tabs via a single `SharedWorker`.
- **Pubsub broker** (`services/pubsub`) for fan-out by topic, behind a
`Broker` interface. `MemoryBroker` is the default (single process); a
Redis backend is available for multi-process setups, configured via
`[websocket].PUBSUB_TYPE` / `PUBSUB_CONN_STR`. The internal Gitea queue
was not usable here because it has FIFO/single-consumer semantics.
- **Push-only event production.** Events are emitted by write-triggered
notifiers — `NotificationCountChange`, `PublishStopwatchesForUser`, and
the logout publisher — wired into the existing `notify.Notifier`
interface. No server-side pollers.
- **Typed pub/sub on the client.** `web_src/js/modules/worker.ts` is a
singleton transport; features subscribe per event type via
`onUserEvent('notification-count', cb)` instead of branching on
`event.data.type`.
- **Wire contract** (`UserEventType` union) is shared between the worker
and consumers via `web_src/js/types.ts`, kept in sync with
`services/websocket/events.go`.
- **Client-side periodic polling fallback** kicks in only when the
WebSocket cannot be established (e.g. proxy blocks WS, browser lacks
module-SharedWorker support).

### What's removed

- `modules/eventsource` (SSE manager, run loop, messenger).
- `/user/events` route and `tests/integration/eventsource_test.go`.
- All server-side polling for stopwatches and notification counts.

### Stopwatch multi-tab fix

The navbar stopwatch icon was previously rendered conditionally on `{{if
$activeStopwatch}}`, so tabs loaded before the timer started had no DOM
element to update. The icon and popup are now always rendered (toggled
with `tw-hidden`), and the start/stop/cancel handlers POST silently so
all open tabs reflect the change in real time.

### Deployment note

WebSocket needs the upgrade headers to pass through a reverse proxy,
e.g. for nginx:

```nginx
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
```

Without them the WebSocket cannot be established, and after 3
consecutive failed opens the shared worker signals `push-unavailable`:
the notification count and stopwatch fall back to periodic polling on
the existing `[ui.notification]` timeouts. Real-time push is lost, the
features keep working. The reverse-proxy docs need the same note (see
the `docs-update-needed` label).

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Epid <rexmrj@gmail.com>

services/notify/notifier.go

@@ -82,4 +82,8 @@ type Notifier interface {
 	WorkflowRunStatusUpdate(ctx context.Context, repo *repo_model.Repository, sender *user_model.User, run *actions_model.ActionRun)
 
 	WorkflowJobStatusUpdate(ctx context.Context, repo *repo_model.Repository, sender *user_model.User, job *actions_model.ActionRunJob, task *actions_model.ActionTask)
+
+	NotificationCountChange(ctx context.Context, userID int64)
+
+	StopwatchChanged(ctx context.Context, user *user_model.User)
 }

services/notify/notify.go

@@ -416,3 +416,17 @@ func WorkflowJobStatusUpdate(ctx context.Context, repo *repo_model.Repository, s
 		notifier.WorkflowJobStatusUpdate(ctx, repo, sender, job, task)
 	}
 }
+
+// Callers must invoke this after any DB write affecting the user's unread count.
+func NotificationCountChange(ctx context.Context, userID int64) {
+	for _, notifier := range notifiers {
+		notifier.NotificationCountChange(ctx, userID)
+	}
+}
+
+// Callers must invoke this after any stopwatch start/stop/cancel so the user's connected tabs refresh.
+func StopwatchChanged(ctx context.Context, user *user_model.User) {
+	for _, notifier := range notifiers {
+		notifier.StopwatchChanged(ctx, user)
+	}
+}

services/notify/null.go

@@ -219,3 +219,9 @@ func (*NullNotifier) WorkflowRunStatusUpdate(ctx context.Context, repo *repo_mod
 
 func (*NullNotifier) WorkflowJobStatusUpdate(ctx context.Context, repo *repo_model.Repository, sender *user_model.User, job *actions_model.ActionRunJob, task *actions_model.ActionTask) {
 }
+
+func (*NullNotifier) NotificationCountChange(_ context.Context, _ int64) {
+}
+
+func (*NullNotifier) StopwatchChanged(_ context.Context, _ *user_model.User) {
+}